Please read, surfaces across Google ie: Google Shopping Free Listing, if you need guidance on this topic, the below is for Google Shopping Paid Ads.
A very simple question with multiple reasons, we will start with the most common and basic issues you might have with showing products in Google Shopping.
As of 3rd of October 2016, you can no longer view products from countries outside of your own. For example if I’m in France and want to buy a product in the UK and want to use Google Shopping, than you will only see ads from France. Or if you are in a country that does not support Google Shopping, than in this case you will see nothing.
I have explained in more detail about products not being visible in Google Shopping
You can find a full list of which countries Google Shopping supports.
If you do want to see a preview of ads outside your native country, check out this link: http://isearchfrom.com/
Every account when submitting a new data feed or setting up a new account will have an initial manual and automated review. This process can take up to 3 working days (not Saturday & Sunday). To check the status, you can log in to your Google Merchant Center account and click on Products > Diagnostics or Products > All Products if you want to view individual products
In the diagnostics panel you will see a graph, showing you an overal data of approved, disapproved, under review etc…
Check your diagnostics tab inside Google Merchant, you might find Errors and Warnings. Products > Diagnostics
Check your data feed, there might be a crawling or import errors. Products > Feeds > Click on your feed > Processing
Data Feed Errors are issues that need to be fixed for the product to be considered for reviewing, not fixing this issue might result in an account suspension.
Warnings are issues that help improve the data feed quality.
Check your Google Merchant Center account, if Google Ads is linked.
In the top right corner click on the wrench and click on Account Linking.
If your login email address for Google Merchant Center is the same as your Google Ads Account, then you will see your account number listed under Your Google Ads Account, simply click on link.
If the login details are different, click on the blue plus button and enter in your Google Ads account number, you can find this by logging into the Google Ads Account, and copy the number in the top left or right corner.
Look into your Google Ads campaign and see if you have a campaign with the label product listing, if not you can create a shopping campaign in Google Ads.
Your bids could be too low, I always recommend starting with $0.50 / £0.50 to ensure enough breathing room is established for the CPC
Ensure that what you are searching for is visible in the titles and descriptions. The titles have a high-ranking factor. Google only shows ads for search terms that match or are relevant to the title of the product submitted in the data feed.
You might find that the landing page experience score is very low on your products, read the following article to check what the quality score is of your products for Google Shopping
When adding variants you need to at least have one of the following attributes filled in. If this is not added, you will not gain the full potential.
Make sure your products are not being submitted from other sites or marketplaces such as eBay, Amazon, Etsy. As only 1 listing per business is allowed. You the merchant is the business.
It is strongly recommended to use Google Ads data, instead of visually looking for your own products in Google Shopping. As this negatively affects your ranking. Want to know more, have a read on why searching for your own ads is a bad idea.